The following background information is provided to assist the reader to understand the environment in which the invention will typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless specifically stated otherwise in this document.
Draft gear assemblies are widely used in United States railway industry to provide protection to a railway car by absorbing shocks in both draft and buff conditions.
The draft gears are installed in alignment with a railway car center sill and typically include a housing having a closed rear end and an open front end, a compressible cushioning means positioned within the closed rear end and a friction cushioning element adopted in the front end and associated with a spring release mechanism for continuously urging the friction cushioning element outwardly from the compressible cushioning means thereby releasing such friction cushioning element after compression of such draft gears.
The commonly used compressible cushioning means is typically either of an all spring configuration as taught by U.S. Pat. Nos. 5,152,409, 5,590,797 and 6,488,162 or of a spring and hydraulic assembly combination as taught by U.S. Pat. No. 3,368,698.
Lately, a new type of draft gear assembly utilizing an elastomeric energy absorbing material has been introduced into service. A particular type of such draft gear assembly, called Twin-Pac, taught by U.S. Pat. No. 6,446,820, utilizes elastomeric pad stacks for absorbing shocks in both draft and buff conditions.
This design includes a rear compressible resilient member disposed between a rear follower and a yoke portion and a front compressible resilient member disposed within the yoke and associated with a coupler follower. A center rod extends through the yoke portion, through the rear compressible resilient member and through the rear follower. Both the front and rear compressible resilient members are comprised of a stack of elastomeric pads. The design does not utilize a steel housing and steel friction members, thus resulting in reduced draft gear assembly weight.
In draft, only the front stack of pads is loaded, as yoke is moving toward the coupler follower which is abutting a pair of front stops, however in buff, both stacks of pads are loaded as the coupler follower is first moving toward the yoke and further causing the yoke to move in a direction of the rear follower which is abutting a pair of rear stops. This design allows for a different amount of draft and buff travel but still falls within the 6.5-inches of total travel required by the AAR M-901 specification. The integral yoke portion can be configured for interface with an E-coupler, an F-coupler, or a rotary dump type coupler.
Testing of the Twin-Pac draft gears according to paragraphs 6.0 and 9.0 of AAR standard M-901G-00 and involving two 70-ton nominal capacity covered hopper cars loaded to maximum gross weight of 220,000 pounds with dry sand resulted in draft gears failure at a speed being over 8 miles per hour and coupler force approaching 1,000,000 pounds. Event though the speed and force parameters exceed requirements of the AAR standard M-901G-00, it is well known that they are applicable during operation on the track.
It has been determined that the draft gear assembly failure was due to over-compression of the elastomeric pad stacks.
Since the design offers an advantage of reduced weight, it is desirable to provide an overtravel protection preventing over-compression of the elastomeric pad stacks and enabling a more reliable operation of the draft gear assembly.
U.S. Pat. No. 5,312,007 to Kaufhold et al. entitled “Slackless Railway Coupler With Draft/Buff Gear” discloses use of the rear compressible resilient member to absorb impact energy during buff conditions in combination with a plurality of guide bars for limiting the compression of the rear compressible resilient member. However, the slackless type coupler does not utilize the yoke as disclosed in U.S. Pat. No. 6,446,820 and does not utilize the front resilient compression member to absorb impact energy in draft conditions.
Therefore, it is desirable to provide an overtravel protection preventing over-compression of the elastomeric pad stacks in both buff and draft conditions.
It is further desirable to achieve such overtravel protection in the most economical method of retrofitting existing draft gears.